There are numerous applications in which the distance from an instrument to a surface must be measured to a high precision. For example, U.S. Pat. No. 4,926,452 to Baker, et al. discloses a system in which an x-ray based imaging system having a very shallow depth of field is used to examine solid objects such as printed circuit cards. The shallow depth of field provides a means for examining the integrity of a solder joint without interference from the components above and below the solder joint. The material above and below the solder joint is out of focus, and hence, contributes a more or less uniform background.
To provide the needed selectivity, the depth of field of the imaging system must be less than 2 mils. Unfortunately, the surface variations on the printed circuit card exceed this tolerance. To overcome this drawback, the surface of the printed circuit card is mapped using a laser range finder. The detailed map is used to position the circuit card with respect to x-ray imaging system such that the component of interest is in focus even when the card is translated from one field of interest to another.
Prior art laser ranging systems can provide the required topological map of the surface provided the surface is free of imperfections that have dimensions of the order of those of the diameter of the laser beam. Two types of ranging systems are commercially available. Both types operate by illuminating the point on the surface with a collimated beam of light from a laser. In the first type of system, the laser beam strikes the surface at right angles to the surface and illuminates a small spot on the surface. The illuminated spot is imaged onto an array of detectors by a lens. The distance from the laser to the surface determines the degree to which the illuminated spot is displaced from the axis of the lens. As a result, as the distance changes, the image of the spot moves along the array of detectors. The identity of the detector on which the projected spot falls provides the information needed to determine the distance to the point on the surface. In this type of system, an imperfection that is larger than the laser beam at the point of measurement will result in an error that can be as large as the height of the imperfection.
In more sophisticated versions of this type of system, the image of the spot falls on more than one detector. The detection circuitry computes the center of the image to provide a more precise distance determination. Here, imperfections in the surface that distort the image on the detector array will also cause errors even though the height of the imperfections is insufficient to cause an significant distance error.
The second type of system assumes that the surface is flat and reflective. In this type of system, the laser beam is directed at the surface at an oblique angle and reflected from the surface onto the detector array without an imaging lens. The distance is then measured by identifying the detector receiving the reflected light beam. The distance measurement relies on a knowledge of the angle of incidence of the angle of incidence of the laser beam with respect to the surface. If the surface includes an imperfection which has dimensions similar to that of the laser beam, this assumption will not be satisfied, since the surface of the imperfection will determine the angle of incidence. The resulting errors can be much larger than the height of the imperfection in this type of system.
In principle, the problems introduced by such imperfection could be mitigated by increasing the diameter of the laser beam. Unfortunately, the diameter of the laser beam must be kept to a minimum to provide the required accuracy in the range measurement.
Broadly, it is the object of the present invention to provide an improved laser range finding system.
It is a further object of the present invention to provide a laser range finding system that is less sensitive to imperfections in the surface being ranged than prior art laser range finding systems without expanding the laser beam.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.